Characters in One Piece will be as strong as Oda needs them to be for the sake of the plot.
When he wants to hype up the Whitebeard pirates, he'll portray the commanders as equal to the Admirals. When he needs to hype up the Admirals for the final part of the story, he'll portray the Admirals as a level above the Yonko commanders.
Fights in One Piece are dictated by the plot, there's no sense to be made of power-scaling outside of the plot itself. Greenbull, Fujitora, and Akainu are stronger than all of the Yonko commanders, because Oda needs them to be until Luffy/Zora/Sanji/Kid/Law meet them, then once they do, they'll lose. It doesn't even have to make sense how they lose, Oda will pull something out of thin air.